1. Technical Field of the Invention
The present invention relates to a suction unit having a helical intake port for causing suction air to swirl. The invention is intended for use in a diesel engine of the horizontal single-cylinder direct injection type.
2. Description of the Prior Art
The basic construction of a horizontal single-cylinder direct injection type diesel engine is as described below.
Referring to FIG. 5, an intake valve opening 53 and an exhaust valve opening 54 are arranged side by side in a wall of a cylinder head 51 facing a combustion chamber 52, this arrangement being necessary in view of the location of a valve operating mechanism. A suction air inlet 58 opens in one side wall 57 of the cylinder head 51 on the same side with the intake valve opening 53, this being because of the locations of a radiator 55 and an air cleaner 56, and a helical intake port 59 is provided for connecting the suction air inlet 58 with the intake valve opening 53.
In conventional embodiments of this basic construction the intake port 59 was formed to be substantially horizontal as shown in FIG. 5, and this was known to have the following disadvantages.
A. In case the intake port is of the helical type, the linear section of the intake port 59 is bound to be short and the resultant small inertia force of the suction air cannot be ample for the desired swirling thereof.
B. It is theoretically possible to increase the length of the linear section of the intake port to thereby improve its rectifying performance required for good swirling of the suction air but the engine is then bound to be bulky with its cylinder head 51 wider.
C. Besides the insufficient inertia force of the suction air, a swirl-inducing angle of a helical section 60 of the intake port 59 is small, limited to 90.degree., hence the swirl produced is bound to be weak. Moreover, this weak swirl is bound to become progressively weaker for the swirling suction air, after having entered the combustion chamber 52, is led in such a direction 62 that it is further off a center 61 of the combustion chamber 52 as it advances, the swirling stream being attenuated through collision against the peripheral wall of the combustion chamber 52.
The consequence was poor air-fuel mixing, causing imperfect combustion, which results in an insufficient output of the engine as well as an increased fuel loss. Worse, imperfect combustion means an increased risk of air pollution because of inferior quality exhaust gas.
D. It is also possible to increase the swirl-inducing angle of the helical section 60 for producing a more powerful swirl. The powerful swirl so produced, however, is bound to get further off the center 61 of the combustion chamber 52 as it advances in the direction 63, the result being an increased attenuation of the swirling stream through collision against the peripheral wall of the combustion chamber 52. This measure is thus known to further weaken the swirl, instead of strengthening it.